I am aware of Linux mode on FreeBSD, where Linux binaries can seamlessly be executed in FreeBSD.
From the FreeBSD handbook:
In effect, there is a Linux kernel in the FreeBSD kernel. The various underlying functions that implement all of the services provided by the kernel are identical to both the FreeBSD system call table entries, and the Linux system call table entries: file system operations, virtual memory operations, signal delivery, and System V IPC. The only difference is that FreeBSD binaries get the FreeBSD glue functions, and Linux binaries get the Linux glue functions. The FreeBSD glue functions are statically linked into the kernel, and the Linux glue functions can be statically linked, or they can be accessed via a kernel module.
This is essentially an ABI implementation. I'm curious if there is an equivalent ABI implementation in Linux operating systems which would allow the seamless execution of FreeBSD binaries.